$12,000 for a 4kw solar system setup? Are you kidding me? for $12,000 I could get enough Do it your self solar cells to do almost a 10kw solar system and buy the stuff needed to have a complete solar system setup. The key to a solar system is wieing up the batteries right to store alot of Amp Hours and the solar panels to charge them properly. if you brake it up right you can get a bigger solar setup then if you had a big single combined one.
#1 rule in solar is make sure your solar system can store more then enough Amp hours then your going to use.
#2 rule make sure you can keep each battery bank system charged with as little solar panels needed.
#3 rule don't use an inverter that is rated at say 2,000w when you will only be using around 500w or w/e. Always use inverters that is a bit higher then what your going to be using to reduce the wasted power that isn't getting used.